Audi TT Coupe (U.S. Version) Fog Light Modification for Daytime Running Light Operation

The U.S. Version of the Audi TT Coupe (and Roadster) does not come factory equipped with daytime running light capability. Daytime running lights being headlamps or similar, smaller lamps that are on whenever the ignition of the car is on. These lamps then also remain on when the parking lights and the headlights are turned on. I feel daytime running lights add to the safety level and overall appearance of any car. You’re more visible to pedestrians and other motorists and the car looks better in the frontal view. So, the challenge was to determine the best way to add daytime running lights to the TT Coupe. Since the TT is equipped with fog lights that are built in to the headlight cluster the plan was to try to modify the circuitry controlling the fog lights to make them behave like daytime running lights. However, I also needed to retain the normal on/off fog light operation during use of the headlights.

Desired Operation:
The fog light wiring needed to be changed or altered to provide the following desired operation:
1. The fog lights come on whenever the ignition switch is turned on.
2. The fog lights come on regardless of the position of the fog light switch EXCEPT when the headlights are on.
3. The fog lights remain on during the normal travel of the headlight switch from off to parking lights on to headlights on.
4. The fog lights go off when the high beams are turned on, both momentarily and during full on usage.

Design:
I had several competing design objectives at work. I obviously didn’t want to re-wire a major portion of the cars’ lighting system. I wanted to keep the modification as simple as possible yet achieve the desired operation. I wanted to, if possible, make the modification an ADD-ON effort, not a re-work effort. The place to start was with the cars’ light switch and its’ wiring. I needed to fully understand how that works to determine if I could make the modification in the manner needed. The source for that information is the cars’ electrical schematics that I purchased from Dyment Inc., (1-800-544-8021).
After studying the schematics for a couple of hours (some parts are in German), it seemed like there was a simple, add-on way to make this work. The headlight switch, parking light switch and the fog light switch are all combined into the one switch mechanism on the dashboard. Most all the wires I needed access to all converge at the rear of the light switch behind the lower dash panel.
The fog lights are controlled by a relay that is controlled by the position of the fog switch in the light switch assembly. The fog light relay is located in a cluster of relays mounted behind the lower dash panel just to the left of the steering column.
If I could install another relay, in parallel with the existing fog light relay, and control it differently as per my desired operation, then I could get my desired results without having to disconnect or re-work the cars’ wiring.

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